Abstract

We design an extremely compact photonic crystal waveguide spatial mode converter which converts the fundamental even mode to the higher order odd mode with nearly 100% efficiency. We adapt a previously developed design and optimization process that allows these types of devices to be designed in a matter of minutes. We also present an extremely compact optical diode device and clarify its general properties and its relation to spatial mode converters. Finally, we connect the results here to a general theory on the complexity of optical designs.

Figures (8)

Schematic of template structure. Background dielectric rods are indicated with solid circles. Black open circles are the 20 rods that can be added or removed, while the black dashed circles are determined automatically by inversion symmetry. The blue highlighted regions show the waveguides, and the orange highlighted region shows the coupler region where search and optimization are performed.

(a) Ideal mode converter specification. The letters denote different modes. The red and blue arrows represent input and output in each mode, respectively. The gray region represents the converter device. (b) The scattering matrix of the device.

(a) Ideal optical diode specification. Similar to the ideal mode converter, an input even mode from the left is converted to an odd mode on the right, and vice versa. However, an odd mode from the left or an even mode from the right are completely reflected. (b) The scattering matrix of the device.

Schematic of optimized structures. (a) Candidate structure resulting from the combinatorial search. The blue highlighted regions show the waveguides, and the orange highlighted region shows the coupler region where search and optimization are performed. (b) Final rod radius optimized structure of the corresponding structure on the left.

Spectral conversion efficiency of optical diode device. (a) The transmission spectrum from mode A to mode D (black line) and the reflection spectrum for mode C to mode C (red line) for the unoptimized structure shown in Fig. 5(a). (b) The same spectra for the optimized structure in Fig. 5(b).

Final candidate spectra obtained from the combinatorial search over 220 possibilities at frequencies near 0.4 × 2πc/a. The vertical axis shows the conversion efficiency from mode A to mode D, and only spectra for structures with peak efficiency greater than 95% are shown.